524 THE INVERTEBRATA 



together form a bulky mass, filling up the dorsal part of the meso- 

 somatic body cavity. The food passes into the cavity of these to be 

 digested. It consists mainly of insects, which are chewed by the 

 gnathobases and the juices sucked up by the action of the pharynx. 

 The beginning of the short hind gut is marked by the Malpighian 

 tubules. 



The nervous system consists of a supraoesophageal ganglion which 

 supplies the eyes, a large suboesophageal complex which gives 

 branches to all the adult appendages, and two ventral cords which 

 bear ganglia in the last seven segments. 



The sexes are separate and the gonads constitute a network. The 

 spermatozoa are filiform and fertilization is internal, being preceded 

 by a courtship, described in lively fashion by Fabre as danse a deux. 

 Scorpions are viviparous. Sometimes the eggs are rich in yolk and 

 the young develop entirely at its expense ; in Scorpio and other genera 

 the eggs are small and yolk is entirely absent. In this case the young 

 develop in lateral sacs of the uterus, attached to the mother by a kind 

 oi placenta. The young, when hatched, are sometimes carried on the 

 mother's back. 



The earliest scorpions are found in the Silurian, and it is of con- 

 siderable interest that the first genus, Palaeophonus, was a marine 

 animal. It closely resembles the terrestrial scorpions, except in its 

 shorter and broader limbs without claws, and in the absence of 

 stigmata. 



Class EURYPTERIDA 



Extinct aquatic arachnids resembling the scorpions in the number 

 and arrangement of the segments of the adult; the division of the 

 abdomen into meso- and metasoma is not quite so marked ; chelicerae 

 short and three-jointed, chelate; the next four segments bear append- 

 ages which are often similar (but the pedipalps may be chelate); in 

 the last (6th) prosomatic segment the appendages are always larger 

 than the rest and are broad and paddle-shaped; first and second pairs 

 of mesosomatic appendages unite to form the genital operculum ; the 

 first five mesosomatic segments bear indications of leaf-like branchiae ; 

 metasoma ends in a structure (telson) of variable form ; mouth has 

 moved backwards and is surrounded by gnathobases of all the limbs. 

 The great interest of this group lies in its similarity to the scorpions. 

 There was, however, much more variety in external structure in these 

 aquatic arachnids and they sometimes attained a length of six feet. 

 Not only is there fundamental agreement in the segmentation and the 

 division into meso- and metasoma, but also in characters like the shape 

 and usually the size of the chelicerae, and the telson, which in primitive 

 eurypterids has a recurved sting-like form. Slimonia (Fig. 358 B) 



